This might seem a question with a very obvious answer.
But it might be tougher, depending on the situation. Massa has been much critizised in Brazil because of his performance in the last years, and because of his willingness to do whatever the team asks him o do. This might seem a little unfair from an international perspective, but in Brazil they are pretty unforgiving about these things. And they’ve had traumas in he same arena with Rubens Barrichello.
Now Felipe Massa is usually on his top game at Interlagos, where he simply dominantes when he’s in form, even if the car is not necessarily the best on the grid. I can see him doing better than Alonso this weekend.
Still, no reason to believe he would concede victory to Alonso if the situation asked for it.

But what if Alonso were further down and Vettel out of the race, and the Spaniard needed one more position to be WDC? Would Massa let whoever were behind him pass, and everyone else, until Alonso was promoted a position, clinching the championship? In other words, would Massa throw a home race away and give up positions or even the whole race to give Alonso the title? As a team player, that’s what he would be expected to do; but it would be really negative for his public image in Brazil, to give away a home race without the team mate immediately behind. I don’t know if he’d have the guts, but I wish he would see it through, win the race and argue that Alonso could have done it himself, beating whoever was in front of him.

In that las situation described, do you think he should let everyone pass until Alonso is promoted, even if it were not the decisive position necessary, just one closer to it? And would he do it?

I don’t think Webber would drop from P1 through the field (apart from the fact that it’s mathematically impossible for him to gift Vettel the championship that way), and there’s a good reason for it: it’s the driver’s championship, and Vettel and Alonso should be able to win it on their own strength, not through their team mates parking their cars from the race lead.

This IS a given – Massa would yield to give his teammate the WDC and Ferrari the honor of having a WDC. It’s also a given that should Vettel find himself behind Massa, he’s not passing him in one piece… Unfortunately, the same applies to Alonso against Webber, Vergne and Ricciardo but to a lesser extent.